The book was written in 1946, the education system has witnessed great changes.
A. when
B. during which
C. since then
D. since when
网友回答
A. when
B. during which
C. since then
(答案→)D. since when
解析：考查定语从句。since when以为“从那时起”，when时间副词，这里指代时间 1946，相当于since when的意思就是since the year 1946。句意：自从1946年起，教育体系经历了巨大变革。